The present invention relates to a single-vision aspherical spectacle lens to correct eyesight and a processing method thereof.
In general, a spectacle lens is custom-made to meet the customer""s specification. However, it takes long time to process both front and back surfaces after receiving the customer""s order. Therefore, semifinished lens blanks whose front surfaces are finished are stockpiled and a back surface of the selected semifinished lens blank is processed according to the customer""s specification in order to shorten delivery times. The lens whose front and back surfaces are processed is an uncut finished lens. The uncut finished lens is edged according to a shape of a frame to obtain an edged lens.
During the processing of the spectacle lens, it is necessary to define a framing reference point that is a reference point when the lens is installed on a frame. The framing reference point is coincident with a pupil position of a user when the spectacle lens is installed on a frame. The framing reference point is coincident with an optical center and is located on an optical axis when the lens does not include a prism for correcting hereophoria (visual axes are deviated from each other during idle period). Further, when the lens includes the prism, the framing reference point is coincident with a prism reference point at which the design prismatic power is obtained.
A semifinished lens blank 1 has a circular outline shape as shown in FIG. 7. In general, a back surface of the lens blank 1 is processed under the condition where a geometrical center 2 is coincident with the framing reference point 3 to ease the processing. The uncut finished lens is, as shown in FIG. 8, edged according to a shape of a frame to be an edged lens 4. The framing reference point 3, which is coincident with the geometrical center 2, will be in agreement with a pupil position 5 of a user.
However, when the frame size is too large or an interpupillary distance is too short, the framing reference point 3 is largely decentered from a boxing center 4xe2x80x2 of the edged lens 4 as shown in FIG. 9. The boxing center 4xe2x80x2 is the center of a recrangle that is circumscribed around the edged lens 4. In such a case, when the framing reference point 3 is coincident with the geometrical center 2 of the semifinished lens blank 1 as described above, the planed shape of the edged lens 4 will be protruded from the semifinished lens blank 1, which makes the processing impossible.
Therefore, a decentering processing is known as a prior art to process spherical lenses whose front and back surfaces are spherical. In the decentering processing, the semifinished lens blank 1 is processed under the condition where the framing reference point 3 is decentered from the geometrical center 2 of the semifinished lens blank 1 as shown in FIG. 10. As a result, the planed shape of the edged lens 4 will remain within the semifinished lens blank 1 even if the framing reference point 3 is decentered from the boxing center 4xe2x80x2 of the edged lens 4.
During cutting or grinding process in the decentering processing, as shown in FIG. 11, the semifinished lens blank 1 is attached to a blocking jig 6 of a processing device, and a prism spacer 10 having a wedge shape is inserted between the blocking jig 6 and a rotating member (not shown) to incline the front surface 1a of the semifinished lens blank 1. In another example, the semifinished lens blank 1 is attached to the blocking jig 6 such that the geometrical center 2 thereof is decentered from the rotation axis 7 as shown in FIG. 12.
The back surface 1c of the uncut finished lens 1xe2x80x2 processed by the decentering processing is shown as broken lines in FIGS. 11 and 12. The framing reference point 3 is decentered from the geometrical center 2 of the uncut finished lens 1xe2x80x2. When the spectacle lens under the processing is a spherical lens, since the optical axis, which is perpendicular to both of the front and back surfaces 1a and 1c of the uncut finished lens 1xe2x80x2, intersects the front surface 1a at the framing reference point 3, the optical performance of the decentering lens processed by the decentering processing is equal to that of the non-decentering lens whose framing reference point 3 is coincident with the geometrical center 2 of the uncut finished lens 1xe2x80x2.
On the other hand, when the spectacle lens under the processing is an aspherical lens whose front surface is aspherical, the situation becomes different. As shown in FIG. 13, a semifinished lens blank 11 for an aspherical lens has a front surface 11a that is finished as a rotationally symmetrical aspherical surface and a back surface 11b. The back surface 11b is processed to be a spherical surface or a toric surface to obtain an uncut finished lens. The symmetry axis 12 of the aspherical front surface 11a intersects the front surface 11a at the geometrical center 13 of the semifinished lens blank 11. In order to reduce cost by limiting the number of molding dies, there was no other choice but to conform the symmetry axis 12 to the geometrical center 13.
However, since the above-described conventional aspherical lens is designed to deliver the best optical performance under the condition where the symmetry axis 12 of the aspherical surface 11a intersects the framing reference point 3 that is coincident with the optical center, if the symmetry axis 12 is decentered from the framing reference point 13, the optical performance will be significantly degraded.
Namely, if the conventional semifinished lens blank 11 for the aspherical lens is processed by the decentering processing that is same as for the spherical lens, an uncut finished lens 11xe2x80x2 as shown in FIG. 14 will be formed. Since the optical axis 16 that is perpendicular to both of the front and back surfaces 11a and 11c of the uncut finished lens 11xe2x80x2 and intersects the framing reference point 15 will be decentered from the symmetric axis 12 that intersects the geometrical center 13 of the uncut finished lens 11xe2x80x2, the optical performance will be significantly degraded.
FIGS. 15 and 16 are graphs showing average refractive power error and astigmatism within 50 degrees of visual angle, respectively, of the conventional aspherical lens whose framing reference point 15 is located on the symmetric axis 12 of the aspherical surface 11a. On the other hand, FIGS. 17 and 18 are similar graphs of the aspherical lens whose framing reference point 15 is decentered from the symmetric axis 12 as shown in FIG. 14. Analysis of these graphs shows that the decentering processing is virtually impossible because of the large aberrations.
Accordingly, the conventional aspherical lens employed for a large-size frame cannot be processed by the decentering processing, which requires a lens blank of large size.
It is therefore an object of the present invention to provide an aspherical spectacle lens, which is capable of using a lens blank of small size even if a lens is employed for a large-size frame and is capable of keeping high optical performance. A further object of the present invention is to provide the processing method of the above aspherical spectacle lens.
For the above object, according to the present invention, there is provided an improved single-vision aspherical spectacle lens to correct eyesight, which includes:
a front surface; and
a back surface,
wherein at least one of the front and back surface is aspherical, a framing reference point that is coincident with a pupil position of a user when the lens is installed on a frame is decentered from a geometrical center of an uncut circular lens (a semifinished lens blank or an uncut finished lens).
With this construction, a lens blank of small-size can be employed for manufacturing a spectacle lens for a large-size frame by deviating the framing reference point from the geometrical center of the lens blank. Further, when the symmetric axis of the aspherical surface intersects the aspherical surface at the framing reference point, the optical performance can be kept high.
In the case when a semifinished lens blank whose front surface is finished is employed, it is desirable that the front surface is spherical and the back surface is processed as an aspherical surface according to a required specification.
The aspherical surface may be a rotationally-symmetrical surface when the lens does not include a cylindrical power to correct astigmatism of an eye. When a cylindrical power is required, the aspherical surface may be symmetric with a pair of planes of symmetry that are perpendicular to each other. The symmetric axis for the rotationally-symmetrical surface is a rotation axis, and that for the surface symmetric with a pair of planes of symmetry is an intersection line of the planes.
Further, the processing method according to the present invention comprises:
attaching a semifinished lens blank whose front surface is finished to an NC machine tool; and
cutting or grinding a back surface of the semifinished lens blank to be an aspherical surface,
wherein the semifinished lens blank is attached to the NC machine tool such that the front surface is not inclined with respect to the machine coordinate of the NC machine tool.
With the above method, an operator is able to attach the semifinished lens blank to the NC machine tool in the same manner as a normal lens whose framing reference point is coincident with the geometrical center without any confusion. Further, when an NC lathe is used to process the back surface, it is desirable to rotate the semifinished lens blank about an axis that intersects the geometrical center during the processing in order to stabilize the rotation torque. Since the front surface is not inclined with respect to the machine coordinate, the target shape of the back surface should be inclined with respect to the machine coordinate for the decentering processing. Thus, the processing method is desirable to include a step for transforming the target shape of the back surface defined in the predetermined coordinate system to that in the machine coordinate thereby creating NC data for the NC machine tool.